In a magnetic resonance imaging apparatus (MRI apparatus), a detecting coil for receiving a magnetic resonance signal is located in an imaging space within a gantry together with a subject. The magnetic resonance signal detected by the detecting coil is generally transmitted from the imaging space to an main unit (hereinafter referred to as a control/imaging unit) via a cable extending to the outside of the gantry. The control/imaging unit subjects the magnetic resonance signal to data processing including image reconstruction processing and thereby images information regarding the subject.
In such a general configuration, the cable is often an obstacle. As disclosed in Jpn. Pat. Appln. KOKAI Publication No. 5-261083, it has been conceived that, in order to avoid such a disadvantage, the magnetic resonance signal is digitized by an analog-digital converter (ADC) in a probe unit that includes the detecting coil called an RF probe, and then converted into a radio signal and wirelessly transmitted to the control/imaging unit by a data transmitter.
A sampling rate for digitizing an RF signal has to be twice the frequency of an RF echo signal or more. Thus, if the RF echo signal is digitized as it is, a high transmission data rate is required for the data transmitter, and the power consumption of the whole probe unit is increased accordingly. As the probe unit that is designed for wireless use is activated by electric power supplied from a power source having a limited power capacity such as a secondary battery, the power consumption of the probe unit is desirably as low as possible.
It is therefore conceived to decrease the frequency of the RF echo signal by frequency down-conversion to decrease the sampling rate so that the transmission data rate required for the data transmitter may be decreased accordingly. When the frequency down-conversion is used, the frequency characteristic of a filter at the subsequent stage of a mixer is designed to be fixed, and the frequency of a local signal to be supplied to the mixer is desirably variable. However, it is not preferable to install, in the probe unit, sophisticated hardware that satisfies a high frequency resolution required for a frequency varying function of the MRI.
Under such circumstances, it has been requested to dispense with the frequency varying mechanism on the coil side, and at the same time to alleviate the data rate requirement of the data transmitter to reduce the load on the probe unit.